1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention is a papermaker""s fabric of the on-machine-seamable variety, such as an on-machine-seamable press fabric for the press section of a paper machine.
2. Description of the Prior Art
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water drains from the slurry through the forming fabric during this process, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed web proceeds from the forming section to a press section, which includes a series of press nips. There, the web passes through the press nips supported by a press fabric, or, as is often the case, between two press fabrics. In the press nips, the web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulose fibers in the web to one another to turn it into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speed. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Referring, for the moment, specifically to press fabrics, it should be recalled that, at one time, press fabrics were supplied only in endless form. This is because a newly formed paper sheet is extremely susceptible to marking in the press nip by any nonuniformity in the press fabric or fabrics. An endless, seamless fabric, such as one produced by the process known as endless weaving, has a uniform structure in both its longitudinal (machine) and transverse (cross-machine) directions. A seam, such as a seam which may be used to close the press fabric into endless form during installation on a paper machine, represents a discontinuity in the uniform structure of the press fabric. The use of a seam, then, greatly increases the likelihood that the paper sheet will be marked in the press nip.
It follows, then, that the seam region of any workable on-machine-seamable, or OMS(copyright), press fabric must behave under load, that is, under compression in a press nip, like the rest of the press fabric, and must have the same permeability to water and to air as the rest of the press fabric, in order to prevent the paper product being manufactured from being marked by the seam region. OMS(copyright) is a registered trademark of Albany International Corp.
Despite the considerable technical obstacles presented by these requirements, it remained highly desirable to develop an on-machine-seamable press fabric because of the comparative ease and safety with which it could be installed on a press section. Ultimately, these obstacles were overcome with the development of press fabrics having seams formed by providing seaming loops on the crosswise edges of the two ends of the fabric. The seaming loops themselves are formed by the machine-direction (MD) yarns of the fabric. A seam is formed by bringing the two ends of the press fabric together, by interdigitating the seaming loops at the two ends of the fabric, and by directing a so-called pin, or pintle, through the passage defined by the interdigitated seaming loops to lock the two ends of the fabric together. Needless to say, it is much easier and far less time-consuming to install an on-machine-seamable press fabric, than it is to install an endless press fabric, on a paper machine.
There are several methods for producing a press fabric that can be joined on the paper machine with such a seam. One method is to flat-weave the fabric, in which case the warp yarns are the machine-direction (MD) yarns of the press fabric. To form the seaming loops, the warp ends are woven some distance back into the fabric body in a direction parallel to the warp yarns. Another technique, far more preferable, is a modified form of endless weaving, which normally is used to produce an endless loop of fabric. In modified endless weaving, the weft, or filling, yarns are continuously woven back and forth across the loom, in each passage forming a loop on one of the edges of the fabric being woven by passing around a loop-forming pin. As the weft yarn, or filling yarn, which ultimately becomes the MD yarn in the press fabric, is continuous, the seaming loops obtained in this manner are stronger than any that can be produced by weaving the warp ends back into the ends of a flat-woven fabric. In still another method, a fabric is woven endless, and the endless loop of fabric of fabric thereby obtained is flattened and given the form of two fabric layers joined to one another at two widthwise ends of the flattened loop. One or more widthwise yarns are then removed from each of the two widthwise ends to produce a short gap defined by the freed, that is, the newly unwoven portions of, lengthwise yarns at each end. These unwoven portions of the lengthwise yarns are then used as seaming loops when the two widthwise ends are brought together as described above.
Generally, the manufacture of an on-machine-seamable press fabric includes the attachment of a staple fiber batt to one or both of its two sides. The attachment may be effected by a process called needling (fiber locking) or by hydroentangling, while the on-machine-seamable fabric is in endless form. Once the desired amount of staple fiber batt has been attached, the loop-forming pin or pintle is removed to place the press fabric into flat, or open, form for shipment and eventual installation on a paper machine. At that time, the staple fiber batt must be cut in the vicinity of the seam to completely separate the two ends of the press fabric from one another. Often, the staple fiber batt is cut in a manner that enables it to form a flap over the seaming loops when the press fabric is rejoined into endless form. For this reason, the two ends of the press fabric are often referred to as the xe2x80x9cflapxe2x80x9d end, which has the flap of staple fiber material extending over and beyond the seaming loops, and the xe2x80x9cno-flapxe2x80x9d end, which has a space, adjacent to its seaming loops, into which the flap on the other end fits when the fabric is joined into endless form. It should be noted that, when the fabric is installed on a paper machine, the orientation of the fabric is such that the xe2x80x9cflapxe2x80x9d end will lead the xe2x80x9cno-flapxe2x80x9d end through the press nip or nips to prevent the flap from wearing away too quickly.
On the other side, the so-called xe2x80x9crollxe2x80x9d side, of the press fabric, however, some staple fiber batt may be removed from the seaming loops to facilitate the subsequent passage of a pintle therethrough. The removal of this generally small amount of staple fiber batt, nevertheless, makes the seam region slightly more permeable to air and water than the rest of the press fabric. This difference in permeability, or flow resistance, perhaps ever so slight, is enough to cause sheet marking in some situations.
Several approaches have been taken toward solving this problem. One approach involves the use of stuffer yarns with the pintle when the press fabric is being joined into endless form on the paper machine. In another approach, a press fabric comprises two on-machine-seamable base fabrics, one fitting inside the endless loop formed by the other, the two base fabrics being laminated to one another during the needling process. The seam regions of the inner and outer base fabrics are offset slightly with respect to one another, so that the seam region of each will coincide with a non-seam region of the other. Once the desired amount of staple fiber batt has been attached to the inner and/or outer surfaces of the laminated base fabrics, the loop-forming pin or pintle of each on-machine-seamable base fabric is removed to place the press fabric into flat form for shipment and eventual installation on a paper machine. At that time, the staple fiber batt must be cut in the vicinity of the seam in the outer of the two on-machine-seamable base fabrics to completely separate the two ends of the press fabric from one another. As above, the staple fiber batt may be cut in a manner that enables it to form a flap over the seaming loops when the press fabric is rejoined into endless form. Some of the staple fiber batt may also be removed from the seaming loops of both the inner and outer on-machine-seamable base fabrics to facilitate the subsequent passage of pintles therethrough.
In yet another approach, disclosed in U.S. Pat. Nos. 5,476,123 and 5,531,251 to Rydin, one or more extra CD yarns are woven with the seaming loops of at least one end of a base fabric of an on-machine-seamable press fabric. The extra yarn or yarns are woven only with those portions of the seaming loops that are on one side of the fabric, that side preferably being the paper-supporting side. The extra CD yarn or yarns form an extension of the CD yarn system of the base fabric at the seaming loop or loops, conforming the seam region more closely to the rest of the base fabric, so that staple fiber batt will be better anchored to the seam region and so that the possibility of sheet marking by the seam region will be reduced.
Typically, one extra CD yarn is woven with those portions of the seaming loops on the paper-supporting side of the fabric in a plain weave. While this has been found to reduce the marking of the paper sheet being manufactured by the seam region, this benefit has been accompanied by the drawback that the extra CD yarn woven in a plain weave tends to raise those seaming loops where it weaves under the portions thereof on the paper-supporting side of the fabric, and to lower those seaming loops where it weaves over the portions thereof on the paper-supporting side of the fabric. In other words, the vertical positions of alternate seaming loops lie in one plane, while those in between lie in another plane displaced slightly in a direction perpendicular to the plane of the fabric. As a consequence, when the fabric is to be joined into endless form, the interdigitated seaming loops will never line up perfectly, making the task of threading a pintle through the passage formed by the interdigitated seaming loops more difficult. The smaller the diameter of the seaming loops, the more serious will be the difficulty of closing the fabric into endless form, as the diameter of the extra CD yarn will represent a larger percentage of the diameter of the seaming loops as a whole, leaving proportionally less area available for the pintle. The use of an extra CD yarn of lighter weight, woven at lower tension on the loom, reduces the plane difference somewhat, but once the extra CD yarn is too light, it cannot fill its intended function.
The present invention provides a solution to the foregoing problem.
Accordingly, the present invention is an on-machine-seamable papermaker""s fabric which comprises a system of machine-direction (MD) yarns and a system of cross-machine-direction (CD) yarns. The MD yarns are interwoven with the CD yarns in a preselected weave pattern to form the papermaker""s fabric in a rectangular shape with a length, a width, two lengthwise edges, two widthwise edges, a first side and a second side. The MD yarns form seaming loops along each of said two widthwise edges. The seaming loops also have a first side and a second side coextensive with the first and second sides of the papermaker""s fabric. When the papermaker""s fabric is woven by a modified endless weaving technique, the MD yarns extend back and forth continuously for its length between its two widthwise edges.
At least two additional CD yarns, not part of the system of CD yarns disclosed above, are interwoven with at least one of the first and second sides of the seaming loops at one of the two widthwise edges of the papermaker""s fabric in a leno weave. The leno weave reduces or eliminates any difference in the vertical position of adjacent seaming loops, locks each seaming loop in position, and maintains them in a desired orientation with their planes perpendicular to that of the fabric.
The present invention will now be described in more complete detail, with frequent reference being made to the figures identified as follows.